On Wed, Jun 24, 2020 at 04:32:28PM +0800, Shaokun Zhang wrote:
> get_file_rcu_many, which is called by __fget_files, has used
> atomic_try_cmpxchg now and it can reduce the access number of the global
> variable to improve the performance of atomic instruction compared with
> atomic_cmpxchg.
>
> __fget_files does check the @f_mode with mask variable and will do some
> atomic operations on @f_count, but both are on the same cacheline.
> Many CPU cores do file access and it will cause much conflicts on @f_count.
> If we could make the two members into different cachelines, it shall relax
> the siutations.
>
> We have tested this on ARM64 and X86, the result is as follows:
> Syscall of unixbench has been run on Huawei Kunpeng920 with this patch:
> 24 x System Call Overhead 1
>
> System Call Overhead 3160841.4 lps (10.0 s, 1 samples)
>
> System Benchmarks Partial Index BASELINE RESULT INDEX
> System Call Overhead 15000.0 3160841.4 2107.2
> ========
> System Benchmarks Index Score (Partial Only) 2107.2
>
> Without this patch:
> 24 x System Call Overhead 1
>
> System Call Overhead 2222456.0 lps (10.0 s, 1 samples)
>
> System Benchmarks Partial Index BASELINE RESULT INDEX
> System Call Overhead 15000.0 2222456.0 1481.6
> ========
> System Benchmarks Index Score (Partial Only) 1481.6
>
> And on Intel 6248 platform with this patch:
> 40 CPUs in system; running 24 parallel copies of tests
>
> System Call Overhead 4288509.1 lps (10.0 s, 1 samples)
>
> System Benchmarks Partial Index BASELINE RESULT INDEX
> System Call Overhead 15000.0 4288509.1 2859.0
> ========
> System Benchmarks Index Score (Partial Only) 2859.0
>
> Without this patch:
> 40 CPUs in system; running 24 parallel copies of tests
>
> System Call Overhead 3666313.0 lps (10.0 s, 1 samples)
>
> System Benchmarks Partial Index BASELINE RESULT INDEX
> System Call Overhead 15000.0 3666313.0 2444.2
> ========
> System Benchmarks Index Score (Partial Only) 2444.2
>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> Signed-off-by: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
> ---
> include/linux/fs.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3f881a892ea7..0faeab5622fb 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -955,7 +955,6 @@ struct file {
> */
> spinlock_t f_lock;
> enum rw_hint f_write_hint;
> - atomic_long_t f_count;
> unsigned int f_flags;
> fmode_t f_mode;
> struct mutex f_pos_lock;
> @@ -979,6 +978,7 @@ struct file {
> struct address_space *f_mapping;
> errseq_t f_wb_err;
> errseq_t f_sb_err; /* for syncfs */
> + atomic_long_t f_count;
> } __randomize_layout
> __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
Hmm. So the microbenchmark numbers look lovely, but:
- What impact does it actually have for real workloads?
- How do we avoid regressing performance by innocently changing the struct
again later on?
- This thing is tagged with __randomize_layout, so it doesn't help anybody
using that crazy plugin
- What about all the other atomics and locks that share cachelines?
Will
